Nowadays predominantly microlithographic projection exposure methods are used for producing semiconductor components and other finely structured components, e.g. masks for microlithography. In this case, use is made of masks (reticles) or other patterning devices which carry or form the pattern of a structure to be imaged, e.g. a line pattern of a layer of a semiconductor component. The pattern is positioned in a projection exposure apparatus between an illumination system and a projection lens in the region of the object plane of the projection lens and illuminated with an illumination radiation provided by the illumination system. The radiation altered by the pattern passes as projection radiation through the projection lens, which images the pattern onto the substrate to be exposed, which is coated with a radiation-sensitive layer and the surface of which lies in the image plane of the projection lens, said image plane being optically conjugate with respect to the object plane.
In order to be able to produce ever finer structures, in recent years optical systems have been developed which operate with moderate numerical apertures and achieve high resolution capabilities substantially through the short wavelength of the used electromagnetic radiation from the extreme ultraviolet range (EUV), in particular having operating wavelengths in the range of between 5 nm and 30 nm. In the case of EUV lithography having operating wavelengths of around 13.5 nm, for example given image-side numerical apertures of NA=0.3, it is theoretically possible to achieve a resolution of the order of magnitude of 0.03 μm in conjunction with typical depths of focus of the order of magnitude of approximately 0.15 μm.
Radiation from the extreme ultraviolet range cannot be focused or guided with the aid of refractive optical elements, since the short wavelengths are absorbed by the known optical materials that are transparent at higher wavelengths. Therefore, mirror systems are used for EUV lithography. One class of EUV mirrors operates at relatively high angles of incidence of the incident radiation, that is to say with grazing incidence according to the principle of total reflection. Multilayer mirrors are used for normal or almost normal incidence. Such a mirror (EUV mirror) having a reflective effect for radiation from the EUV range has a substrate, on which is applied a multilayer arrangement having a reflective effect for radiation from the extreme ultraviolet range (EUV) and having a large number of layer pairs comprising alternately low refractive index and high refractive index layer material. Layer pairs for EUV mirrors are often constructed with the layer material combinations molybdenum/silicon (Mo/Si) or ruthenium/silicon (Ru/Si).
It is known that the reflectivity or reflectance of multilayer mirrors is greatly dependent on the angle of incidence and on the wavelength of the impinging EUV radiation. A high maximum value of the reflectivity can be achieved if the multilayer arrangement substantially consists of a periodic layer sequence having a multiplicity of identical layer pairs. In that case, however, a relatively low value of the full width at half maximum (FWHM) of the reflectivity curve results both in the case of the dependence of the reflectivity on the angle of incidence and in the case of the dependence of the reflectivity on the wavelength. The prior art discloses examples of the angle of incidence dependence and wavelength dependence of the reflectivity of conventional multilayer mirrors.
In optical systems for the EUV range having a relatively high numerical aperture, for example in projection lenses for EUV microlithography, relatively high variations in the angle of incidence can occur, however, at certain positions in the beam path. In this respect, EUV mirrors are required which have a reflectance that varies only little over the angle-of-incidence range that respectively occurs. Numerous proposals have already been made for the construction of such multilayer mirrors that are broadband in terms of the angle-of-incidence range.
The article “EUV multilayer mirrors with tailored spectral reflectivity” by T. Kuhlmann, S. Yulin, T. Feigl and M. Kaiser in: Proceedings of SPIE Vol. 4782 (2002), pages 196 to 203, describes a special layer construction of EUV mirrors having a broadband effect. The multilayer arrangement comprises a plurality of layer groups each having a periodic sequence of at least two individual layers—forming a period—of different materials. The number of periods and the thickness of the periods of the individual layer groups decrease from the substrate toward the surface. One exemplary embodiment has three different layer groups. What is intended to be achieved by this layer construction is that, firstly, the peak wavelengths of the reflection maxima of the respective layer groups from the substrate toward the surface are shifted to shorter wavelengths, such that a wider reflection peak of the overall system is produced by the superimposition of the reflection of the individual layer groups. Secondly, all of the layer groups can contribute approximately identically to the reflectivity of the overall system. In this way, it is possible to achieve an almost constant reflectivity over a large wavelength range or angular range.
The article “Broadband multilayer mirrors for optimum use of soft x-ray source output” by Z. Wang and A. G. Michette in: J. Opt. A: Pure Appl. Opt. 2 (2000), pages 452-457, and the article “Optimization of depth-graded multilayer designs for EUV and X-ray optics” by Z. Wang and A. G. Michette in: Proceedings of SPIE Vol. 4145 (2001), pages 243-253, indicate examples of EUV mirrors having a broadband effect in which the broadband character is achieved by virtue of the fact that the layer thicknesses of the individual layers of the multilayer coating vary individually in the depth direction of the multilayer arrangement as a result of an optimization process. Such multilayer arrangements having a stochastic sequence of individual layers optimized by way of a simulation program are also designated as “depth-graded multilayers”. The production of such multilayer arrangements can be difficult, since layers having many different layer thicknesses have to be produced successively in a coating process.
The prior art discloses broadband EUV mirrors for normal or almost normal incidence which have a multilayer arrangement having different groups of layer pairs. A surface layer film group is arranged at the radiation entrance side of the multilayer arrangement. Opposite the radiation entrance side there follows an additional layer. This is followed in the direction of the substrate by a deeper group of layer pairs (deep layer film group). In this case, the reflectivity of the surface layer film group is higher than the reflectivity of the deeper layer group near the substrate, and the reflected radiation is phase-shifted on account of the presence of the additional layer such that a reflectivity peak value of the entire multilayer arrangement is lower and the reflectivity around the peak wavelength is higher than in the absence of the additional layer. The optical layer thickness of the additional layer is intended to correspond to approximately one quarter of the wavelength of the EUV radiation (i.e. λ/4) or half of the period thickness of the multilayer arrangement or this value plus an integral multiple of the period thickness. In one exemplary embodiment, the additional layer consists of silicon and is arranged directly adjacent to a silicon layer of a molybdenum/silicon layer pair, such that a silicon layer having a layer thickness which corresponds to at least half the wavelength, i.e. at least λ/2, is situated within the multilayer arrangement.